1. Field of the Invention
The present invention concerns the manufacture of silica powder and in particular a method of manufacturing silica powder with spherical grains specifically intended to be used in the fabrication of optical fiber preforms.
2. Description of the Prior Art
The fabrication of an optical fiber preform is well known in this art. One fabrication method described in U.S. patent application Ser. No. 08/087,060 of Jul. 7, 1993 consists in forming a suspension in water of synthetic silica soot having a specific area of less than 80 m.sup.2 /g and containing 50% to 75% by weight silica, gelling the suspension, drying the gel obtained using microwaves, fractioning the dried gel into silica grains having a diameter between 10 .mu.m and 1 mm and screening between 100 .mu.m and 500 .mu.m. The silica grains obtained in this way are then densified in a furnace at a temperature of approximately 1 300.degree. C. before they are deposited by plasma spraying and melting onto a primary preform, as described, for example, in the article by Le Sergent et al "Preform Technologies for Optical Fibers" in Electrical Communication, Vol. 62, N.sup.o 314, 1988. The object of densifying the grains before making the preform is to increase the rate of deposition of the grains onto the primary preform.
Known as the sol-gel method, this fabrication method unfortunately has the drawback that the grains obtained do not have the required spherical shape; they are usually rectangular or ovoid in shape. This is a problem when the grains are used to make a preform. The preform obtained contains bubbles of gas trapped between the grains. These bubbles make the preform and the resulting optical fiber fragile by creating microstresses that can cause the optical fiber to break. Furthermore, if the grains are not spherical they do not flow well into the plasma flame during fabrication of the preform.
For this reason a primary object of the invention is to provide a device for producing spheroidal grains for application to optical fibers.
Another object of the invention is to provide a method of producing spherical silica grains to be used to make optical fiber preforms.